1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive comprising an offset-nulling amplifier for detecting a back EMF voltage of a voice coil motor.
2. Description of the Prior Art
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
There are times when the servo control system does not have access to the embedded servo sectors yet it is still desirable to control the velocity of the actuator arm. For example, in disk drives wherein the head is parked on a landing-zone of the disk, it is desirable to control the velocity of the actuator arm to unlatch the head during spin-up. In disk drives employing ramp loading/unloading, it is desirable to control the velocity of the actuator arm so that the head is not damaged as it travels off the ramp onto the disk as well as off the disk onto the ramp. Another example is if the servo control system loses servo sector synchronization it is desirable to control the velocity of the actuator arm to facilitate re-synchronizing to the servo sectors.
Prior art techniques for controlling the velocity of the actuator arm when servo sector information is unavailable include using a voltage loop with the detected back EMF voltage generated by the VCM as the feedback. The voltage across the voice coil (the voice coil voltage) comprises a component due to the inductance L of the VCM, a component due to the resistance R of the VCM, and a component due to the velocity of the VCM referred to as the back EMF voltage. If the component due to the resistance R is canceled from the voice coil voltage, at low frequencies Ldi/dt is small leaving the back EMF voltage due to the velocity of the VCM as the dominant component.
A differential operational amplifier (op-amp) has typically been used to detect the back EMF voltage; however, the input offset voltage of a differential op-amp can distort the back EMF measurement. A known technique for nulling the input offset voltage involves shorting the inputs to the differential op-amp during a calibration interval in order to measure the input offset voltage. The stored input offset voltage is then used to adjust the output of the differential op-amp during normal operation. This technique is undesirable, however, since the input offset voltage must be calibrated periodically which can interfere with the normal operation of the disk drive.
There is, therefore, a need to improve the back EMF voltage measurement of a VCM in order to improve the velocity control mode of a disk drive.